Atomic absorption spectrophotometry is utilized to measure the concentration of a particular element in a sample. For example: if one wishes to determine the concentration of copper in a sample, a light source producing one of the characteristic spectral lines of copper is utilized in the spectrophotometer. These are most often hollow cathode or electrodeless discharge lamps where the cathode is of copper, or any other element to be measured. A monochromator usually utilizing a diffraction grating disperses the light from the hollow cathode into a spectrum and the monochromator is adjusted so that the line of interest falls upons a detector, usually a photomultiplier tube. The amount of light falling on the photo tube is measured as a reference.
A sample of material in which one wishes to determine the amount of copper is then introduced into the path of the light from the line source to the monochromator. The sample must be "disassociated" so that the copper atoms are free and not a part of a molecular compound in which case they would not provide their characteristic spectrum. This may be accomplished in an absorption furnace (electrothermic sample atomizer). When the copper atoms are introduced into the light path they absorb light at the same characteristic spectral lines at which the copper atoms in the light source emit light. Thus, at the line of interest, light will be absorbed and less light will fall on the photomultiplier tube. The natural logarithm of the signal from the photo tube when there is no absorbance divided by the signal when the copper is present in the light path to absorb the light is called the absorbance, and from the absorbance the concentration of copper in the sample may be determined.
There is one basic problem in all atomic absorption spectrophotometry. This is the so-called background absorbance, sometimes termed "non-atomic absorption" or "molecular absorption". The problem is that other atoms and molecules in the sample may also absorb light at the characteristic spectral line of interest. This absorption will of course cause an error in the absorbance measured. Various means have been disclosed in the prior art to correct the problem and in general such systems are called "background correction".
The most common form of background correction utilized in commercial atomic absorption spectrophotometers is the continuum source system. In this system light from a broad band light source, that is, one producing a continuous, rather than a line spectrum, is utilized to measure the absorbance of a sample. Another beam is passed through the sample from a characteristic line source. The absorbance is then measured at the line of interest and it is assumed that if one subtracts the absorbance from the continuous line source one will derive the absorbance at the spectral line of interest. There are many problems with such systems. The light from the characteristic line source and the light from the continuous source do not pass through the same path and there may be substantial differences in the concentrations of the sample in the two paths, leading to systematic error.
If a sequential beam system is utilized, wherein the continuous spectrum reference beam is first passed through the sample and thereafter the line source beam, the concentration may vary over time as well as space, again introducing systematic errors.
Another method of background correction has been proposed. This utilizes the Zeeman or Stark effects. In the Zeeman or Stark effects, when a magnetic or an electric field is applied to the sample, the spectral lines characteristic of an atom are split into several spectral lines.
In the normal Zeeman and Stark effects of interest here, a spectral line may be converted into two spectral lines shifted to either side of the normal spectral line, by an amount proportional to the applied field, or into three spectral lines, one at the normal position and two shifted, as aforesaid.
An important feature of the Stark and Zeeman effects is that the split spectral lines do not all have the same polarization and in particular the polarization of the central or normal central line and the shifted spectral lines will be different, thus making it possible to look at the normal line or the shifted lines with a polarization analyzer.
Below are listed a number of prior art patents and publications describing various systems utilizing the Stark or Zeeman effects for background correction in atomic absorption spectrophotometry.
______________________________________ Pat. No. Inventor Date ______________________________________ U.S. PAT. NOS. 3,676,004 Prugger et al 7/11/72 3,811,778 Hadeishi 5/21/74 3,914,054 Hadeishi 10/21/75 3,937,577 Dorsch 2/10/76 4,035,083 Woodriff et al 7/12/77 4,171,912 Ito et al 10/23/79 U.K. PATENTS 918,878 Isaak 2/20/63 918,879 Isaak 2/20/63 1,271,170 Zeiss-Stiftung 4/19/72 1,385,791 Parker and Pearl 2/26/75 1,420,044 US ERDA 1/7/76 ______________________________________